1. Field of the Invention
The stabilization of multitrailer commercial vehicles has been the subject of many studies and inventions. The problem to overcome is the "yaw" of the second and third trailers in combination operation. The yaw is attributable to the distance between the location of the pintle hitch (articulating joint) and the point of rotation at the centerline of the axle of the lead trailer. This distance causes a rearward amplification of any movement of the trailer fifth wheel. The pintle hitch allows the A-dolly supporting the front of the towed trailer to "walk" or "wander" with any road surface variation, such as wear grooves, road seams and the edge of the pavement.
The following quotations which define the field of the invention, and the problems encountered therein which are resolved by this invention, are taken from a report entitled "Improving the Dynamic Performance of Multitrailer Vehicles: A Study of Innovative Dollies - Volume I Technical Report" (hereinafter UMTRI) performed by the University of Michigan Transportation Research Institute under contract with the Federal Highway Administration. The report date is July 1986, and the report number is FHWA/RD-86/161. It is available through the National Technical INformation Service of the U.S. Department of Commerce, 5285 Port Royal Road, Springfield, Va. 22161.
"The Surface Transportation Act of 1982 allows the use of double-trailer combination vehicles nationwide on the designated highway system. It also allows for the use of commercial vehicle widths from 96 in. (2.44 m) to 102 in. (2.59 m). This Act is generally expected to result in a major increase in the number of multitrailer commercial vehicles in use throughout the U.S.. At the same time, pressure for allowing the use of triples is increasing. In light of the fact that multitrailer vehicles are known to suffer from special dynamic characteristics that can limit their stability and emergency maneuverability characteristics vis-a-vis the tractor-semitrailer, these developments have led to concern over the potential for degradation of the safety quality of the U.S. commercial vehicle fleet . . . It is well established in the literature that maneuvering quality of the tractor-semitrailer portion of an A-train doubles combination vehicle is virtually unaffected by the presence of the full trailer, but that, in emergency maneuvers, the second trailer of the doubles suffers from a "crack-the-whip" phenomenon in which the second trailer substantially exaggerates, or amplifies, the motion of the tractor. The major safety consequence of this "rearward amplification" is the premature (sic) rollover of the second trailer. Rearward amplification and the resulting propensity toward rollover of the second trailer is generally recognized as the property of the double which distinguishes (and degrades) its dynamic performance capability from that of the tractor-semitrailer combination vehicle" UMTRI at page 1.
"The major motivation for the use of multiply articulated trains by commercial trucking interests is to obtain a vehicle with high cargo volume which retains the practical benefit of good, low-speed maneuverability. Within the constraints of vehicle height and width laws, more cargo volume is attained by lengthening the vehicle. Generally, as vehicle length increases, so do maneuvering problems, since the magnitude of low-speed offtracking is directly related to vehicle length. However, the offtracking of a vehicle of a given length is generally reduced by the introduction of additional yaw articulation joints. By virtue of these facts, the so-called A-train doubles combination has become a popular commercial vehicle.
"An A-train consists of a tractor-semitrailer pulling one or more conventional full trailers, where a conventional full trailer consists of a semitrailer whose forward end is supported by a dolly which (1) articulates in yaw relative to the semitrailer, (2) is connected to the towing unit by a single pintle hitch, and (3) has one or more axles whose wheels are nonsteering relative to the dolly frame. While the A-train meets the primary need of providing a large-volume vehicle which can be maneuvered relatively easily at low speed, it is also known to be less stable at highway speeds than is the conventional tractor-semitrailer.
"The dynamic stability of the A-train suffers from the phenomenon known as rearward amplification, wherein, in steering maneuvers of relatively high frequency content, trailing units in the trail will tend to experience higher lateral accelerations than their towing unit. Thus, lateral acceleration "amplifies" as one moves rearward along the train, and the rearmost trailer may experience accelerations much larger than those experienced by the driver in the tractor. The most serious safety consequence of the phenomenon is the resulting rollover of the rear trailer. Further, the addition of yaw articulation joints tends to reduce the yaw damping of the vehicle, and the reduction of low-speed offtracking tends to aggravate high-speed offtracking.
"In recent years, the safety-degrading influence of additional articulation joints has become broadly recognized. Nonetheless, the economic motivations for the use of multitrailer trains is so compelling that the use of such vehicles does and will continue to grow. Efforts to improve the safety quality of these vehicles have led, however, to a number of innovative dollies and hitching mechanisms being developed." UMTRI at pages 2 and 3.
The report identifies seven categories of modified A-dollies, and two categories of B-dollies.
"The so-called A-dolly is, of course, the conventional single-drawbar dolly, which connects to the first semitrailer trailer with a single pintle hitch and to the second trailer with a conventional fifth wheel (converter dolly) or with a turntable bearing (turntable dolly). Modified A-dollies . . . are dollies which retain the pintle hitch or any other form of coupling which permits yaw articulating between the dolly and the first trailer. B-dollies, on the other hand, are dollies which practically eliminate yaw motions between the first trailer and the dolly, usually by using rigid double drawbars and two pintle hitch connections." UMTRI at page 8.
The purpose of the B-dolly is to move the Instantaneous Center of Rotation (IC) forward of the pintle hitch connections. With a conventional A-dolly, the IC is the pintle hitch itself. With a B-dolly, the IC is the imaginary point defined by a forward projection of the effective centerlines of the drawbars. As the IC moves forward of the actual towing connection, the high-speed rearward amplification of lateral movement of the towed trailer is reduced substantially.
The use of B-dollies will improve rearward amplification performance and roll stability. However, "(o)fftracking performance is somewhat degraded relative to the A-train, and many practical considerations of cost, frame stressing, incompatibility of existing trailers, etc., serve to limit the applicability and acceptance of the B-train." UMTRI at page 25.
My involvement with the operation of multitrailer combinations began when I became a driver of multi-trailer vehicles, then became Operations Manager for a trucking company, and then was a sales representative for a B-dolly manufacturer. The B-dolly and B-train are considered the most stable way to tow double- and triple-trailer combinations. Most trucking companies are interested in stabilizing multitrailer operation, but object to the cost of retrofitting a fleet. All trailers in the fleet must be reinforced for mounting of two pintle hitches for towing the B-dolly. If the entire fleet is not modified, dispatching becomes almost impossible. One comment made many times was "Why not make a device to stabilize the present A-dolly ?".
The No-Slack Restricted-Drum Stabilizer of this invention is that device. It mounts directly on the tongue of the A-dolly, and provides the same over-the-road stabilization as the B-dolly. The lead trailer must be modified only by mounting two D-ring type fittings on the tail board. If the lead trailer happens not to be modified, there is no dispatching problem, because the modified A-dolly can be towed in the normal manner, by just securing the interconnecting cables of this invention to the dolly tongue.
2. Description of Related Art
My description and comments as to the prior art are given with recognition of certain characteristics that should be present when a device employing a tensioned cable and a restricted drum are used to control or eliminate trailer yaw. The principal force acting upon a trailer is the towing force, which is always there, and which, ideally, will move the trailer along the centerline of travel in a self-correcting manner. The towing force is ordinarily much greater than the yaw force. A sudden removal of the towing force, such as by applying the vehicle brakes allows the yaw force to predominate, sometimes with dire results. External factors cause trailer yaw, and the devices in the cited patents are intended to apply a corrective force to counter the externally-induced yaw. The last thing to do, therefore, when the trailer starts to jackknife, is to apply the brakes, and corrective devices which are activated by application of the brakes are asking for trouble.
Some of the devices in the cited patents rely on springs. The force exerted by a spring is more or less directly proportional to its displacement. When springs are relied on to bring the trailer back in line, they build up a large corrective force which may or may not be required while, at the same time, providing an initially weak countervailing force when the trailer returns rapidly to the centerline. This can possibly cause undesirably large corrections to take place. The corrective force, ideally, should be constant, and should always be available at any position of the trailer.
There is always relative up-and-down motion between the trailer dolly and the towing vehicle. This motion causes slack to occur in the cable of the correcting device. The device must include a means to keep the slack to a minimum or, ideally, to eliminate it altogether.
U.S. Pat. No. 2,612,382 to Landis. This device applies pressure by applying spring force to a captured plate with a backup spring. The spring is used as a return mechanism, so it has to be strong enough to overcome the friction of the pressure plate, resulting in the spring being the tension setter. This results in little force being applied at first movement.
U.S. Pat. No. 3,305,246 to Gonczy et al. This unit uses a block-and-tackle arrangement where the line tension is controlled by a spring.
U.S. Pat. No. 3,338,595 to Bogie. This unit uses a spring to provide cable tension, and a spring-loaded friction plate to restrict cable movement.
U.S. Pat. No. 3,362,727 to Malherbe. This device has a tension spring on each side which reduces the effective force applied.
U.S. Pat. No. 3,379,456 to Bogie. This device works by restricting fluid flow from one side of a cylinder to the other. Slack is taken up by a spring. The first movement is restricted only by the spring until fully compressed, or when pressure on the cylinder is overcome. This unit should work, depending on the size of the spring, but the spring will not give constant tension.
U.S. Pat. No. 3,659,874 to Rendessy. This device has a manual pressure plate for restricting cable movement, but a spring is used to maintain tension. The spring, therefore, provides the first restricting force to the cable.
U.S. Pat. No. 3,680,891 to Rendessy. This device uses a cable wrapped around frictionally-damped rollers, and does not use a spring. There appears to be no way to remove slack caused by up-and-down movement.
U.S. Pat. No. 3,807,764 to Watts et al.. This device works by a brake locking the motion of a disc to which an arm is attached, thus providing a rigid connection between the trailer and the towing vehicle. It appears to be intended for use with only a semitrailer operation. It operates when the vehicle brake is applied, and the driver can also exercise manual control of the braking member.
U.S. Pat. No. 3,837,676 to Rendessy. In this device, a cable is wrapped around several pulleys, one of which may have a restricted movement. Tension is set initially by a toggle. There is no means to compensate for slack cased by up-and-down motion.
U.S. Pat. No. 3,857,586 to Mascuch. This device is intended for use between a tractor and a semi-trailer.
U.S. Pat. No. 3,871,686 to Rendessy. This is similar to ,676 of Rendessy, except none of the pulleys rotate. The same comments apply.
U.S. Pat. No. 3,989,269 to Rendessy. This is a pre-tensioned friction device designed for light--duty towing. There is no provision to take up slack caused by up-and-down motion. It is difficult to see how enough force can be applied to control any movement.
U.S. Pat. No. 4,015,856 to Smith. This is a pre-tensioned device using a cable wrapped around a freely rotating pulley. Shock absorbers attached to the pulley housing offer resistance to induced rotation of the pulley.
U.S. Pat. No. 4,082,309 to Fox. This device comprises a rotor, connected at either side to a fixed bar on the towing vehicle by equal lengths of chain. The rotor is fitted with an electric brake which can be actuated by the driver and by a pressure switch mounted on the side of the trailer at the rear. One would think that the timing of rotor brake actuation would have to be extremely precise to avoid locking the rotor in any but the proper towing position of the trailer.
U.S. Pat. No. 4,469,347 to Gier. This device is used on a unit that has a fifth wheel as part of a tow vehicle. It needs a means of power for the hydraulic motor.
U.S. Pat. No. 3,764,164 to Lankenau. This patent discloses a dolly which is added to a single-rear-axle tractor to enable it to carry heavy loads.
U.S. Pat. No. 3,880,439 to Wolter. This mechanism provides normally rigid alignment of the trailer rear wheels, but has a device which unlocks the wheels and permits self-steer during an actual turn.
U.S. Pat. No. 4,162,082 to Curry. The self-steering dolly of this invention is used under the front of the second trailer. This appears to be a variation of a self-steering B-dolly.
U.S. Pat. No. 4,475,740 to Strick. This patent discloses an articulated double trailer combination with completely different characteristics from the standard highway vehicles in current use.
U.S. Pat. No. 4,557,497 to Rumminger. This patent discloses a second trailer with a retractable towing means for drawing the second trailer flush to the first trailer for convenience in loading.
The patents to Gonczy, Bogie, Malherbe, Rendessy and Fox all appear to be directed toward light-duty devices for use on recreational trailers towed by automobiles and the like.
The patents to Watts and Mascuch appear to be directed toward the connection between a tractor and a semi-trailer, and not to the connection between two trailers, as in the instant invention.